1. Field of the Invention
This invention relates generally to positron emission tomography, a sub-field of the class of medical imaging techniques using ionizing radiation and image reconstruction techniques; and more particularly to devices which use an array of scintillation detectors to detect the annihilation radiation from positron disintegration and use this information to reconstruct an image of the distribution of positron emitting isotope within a body section.
2. Description of the Prior Art
Positron emission tomography is a technique for measuring the concentration of a positron emitting isotope through sectional planes or within a defined volume of the body (for medical research and diagnostic purposes). Normally the isotope is used to label a substance which circulates in the blood and accumulates in certain tissues. The regional concentration of the isotope may be measured if the device is suitably calibrated. The ability to quantitate the regional concentration depends on the spatial resolution of the system. The ultimate limit of the spatial resolution depends on the positron decay process and subsequent production of two annihilation gamma ray photons.
Some isotopes whose nuclei have more protons than neutrons decay by transforming a proton into a positive electron (positron) and a neutrino. The positron is ejected with variable kinetic energy which is lost in many collisions with electrons in the body tissue. The distance travelled depends on the initial energy and the electron density and is typically 0.5 to 2 mm for common isotopes in medical use. The positron finally interacts with an electron, and because they are each other's anti-particles, they annihilate each other normally creating two gamma rays with 511 keV energy which travel in opposite directions. Because the electron and positron are moving at the time of annihilation, the pair of gamma rays, although collinear in the moving frame of reference appear to be slightly angulated by about 1/2 of a degree when imaged by detecting crystals.
The positron range and apparent non-collinearity of the annihilation gamma rays determine the ultimate limits of the spatial resolution. This limit cannot be obtained in practice due to penetration by the gamma rays into the detectors. The 511 keV intrinsic energy of these gamma rays gives them a mean free path of several millimeters even in the densest of materials like lead. Modern imaging systems attempt to minimize the blurring of the detectors by (1) using very dense scintillation crystals such as bismuth germanate, (2) making the crystals very narrow or, (3) using imaging means to determine the location of the interaction of the gamma ray in larger crystals. The detection of an event useful in image formation requires the simultaneous recording of each of the pair of annihilation photons. The nucleus which emitted the positron is assumed to have been on the line joining the points of interaction of the two gamma rays.
When this line is close to a diameter of the circle on which the crystals are disposed, and the crystals are made narrow, the depth of interaction is not important since the crystal could be made deep enough to ensure the gamma rays will almost always be absorbed. When the line is far from being a diameter both of the gamma rays may pass through one or several crystals before being absorbed. This causes a broadening of the coincidence aperture function towards the edges of the field of view.
In modern imaging systems with crystals 3-4 mm wide the spatial resolution is about twice as bad at the edge of the field of view as at the centre. If the crystals are made narrower than 3 mm the blurring due to the positron range and non-collinearity dominate the resolution loss for diametrically opposed crystals, so the resolution improvement gained by using even narrower crystals is not significant. If the crystals are made deeper the resolution loss at the edge is more severe, so there is compromise between deeper crystals which improve total system sensitivity and resolution loss at the edges.